The proposed research is pertinent to developing an understanding of the complexity of the mammalian brain in terms of genetic transcription. Experiments will be done to determine whether a complex population of brain specific messenger RNAs exist, and if so, studies will be conducted to ascertain whether such specificity is controlled at the transcriptional or post-transcriptional level. Several approaches will be applied to determine whether a complex population of nonadenylated mRNA (poly(A) mRNA) exists in addition to the complex poly(A) plus mRNA population. The complexity of various size and abundance classes of heterogeneous nuclear RNA (hnRNA) will be measured. Special emphasis will be placed on small poly(A) hnRNA molecules which are extensively homologous with polyadenylated mRNA. Small class poly(A) hnRNA from neuronal and glial nuclei will be compared. The mRNA content and tissue specificity of high abundance class hnRNA of brain will be investigated. Messenger RNA populations in postnatally developing brain will be compared with that of the adult. Methodology utilized in the proposed research includes DNA/RNA hybridization using a variety of labeled DNA probes, gel electrophoresis, electron microscopy, various chromatographic procedures and various cell fractionation techniques, etc. The proposed studies are intended to provide basic information on such problems as defining the code function of the vastly complex mammalian genome, establishing relationships between genetic transcription (nuclear RNA) and the generation of complex mRNA populations, and defining transcriptional and post-transcriptional control of messenger populations.